Radioactivity is found from 'nuclear test that failed'

However, several other tests carried out by US scientists have failed to determine whether an explosion took place.

And a senior US official said: "The betting is that this was an attempt at a nuclear test that failed.

"We don't think they were trying to fake a nuclear test, but it may have been a nuclear fizzle."

Meanwhile, the United Nations General Assembly has confirmed that South Korea's foreign minister Ban Ki-moon will succeed Kofi Annan as its next secretary-general.

He immediately made peace with North Korea a top priority.

Mr Ki-moon said: "My tenure will be marked by ceaseless efforts to build bridges and close divides. Leadership of harmony not division, by example not instruction, has served me well so far. I intend to stay the course as secretary-general."

Members of the UN Security Council were to vote today on sanctions to be imposed on North Korea.

The draft resolution authorises non-military sanctions but says that any further action would require another UN resolution.